Sulphur dioxide, extrusion

The carbanion generated from deprotonation of the w-carbon atom of sulpholene reacts with aldehydes and ketones to give alcohols. Sulphur dioxide extrusion from the products results in (E)-a-hydroxy-l,3-dienes (equation 72), or dehydration followed by thermal desulphonylation results in trienes117. Dienones can be obtained if the initial condensation is conducted with an aldehyde, followed by oxidation and sulphur dioxide removal117. 

The Diels-Alder adduct of sulpholene and cyclopentadiene is a useful starting material for substituted diene synthesis121. The diene moiety is unmasked by retro-Diels-Alder reaction and sulphur dioxide extrusion under flash vacuum pyrolysis conditions (equations 74 and 75)122,123. 

Ring expansion of the sulphonamide reaction199 (equation 144) demonstrates the ability of a ruptured sulphur-centred free radical to undergo 1,3-Fries type migration200 (equation 145). Sulphur dioxide extrusion may also provide a synthetic route to fi-lactams201 (equation 146). 

Even though sulphur dioxide extrusion from sulpholene derivatives is generally conducted either by heating in a non-polar solvent or in the presence of base, alternative reagents such as LAH, and finely dispersed potassium metal are also used. Syntheses of some tetrasubstituted butadienes were achieved by treating the corresponding sulpholenes with ultrasonically dispersed potassium metal in the presence of water (equation 73) . 

Finally, thermal sulphur dioxide extrusion from c/j,-2,5-disubstituted 2,5-dihydrothiophene 1,1-dioxides, generated by a retro-Diels-Alder reaction, has been used for the synthesis of pellitorine (1) and pipercide (31) [64] (Scheme 19). 

Sulphuranes - see also Dialkoxysulphuranes, Oxysulphuranes as intermediates 406 Sulphur bonding 484-493 Sulphur-containing groups, stereoelectronic effects of 584-594 Sulphur dioxide addition to 215-217 extrusion of 137, 140, 141, 158, 163, 397-402, 801, 805, 962, 1098 Sulphur monoxide, extrusion of 397-402 Sulphur trioxide, reactions of 217, 218 Sultenes, as intermediates 743 Sultines 679, 943 as photolytic products 881, 882... 

Extrusion of sulphur dioxide from cyclic systems leading to dienes has proved to be a synthetically useful reaction112. Thermolysis of cis- and fraw,s-2,5-disubstituted sulp-holenes, which can be readily obtained through regio- and stereoselective alkylation, proceeds in a stereospecific manner affording 1,3-dienes of high stereochemical purity, as predicted by symmetry rules (equations 68 and 69)113. On the other hand, a photochemical process is not completely stereospecific (equation 68)114. 2,5-Dialkylative cyclization... 

A novel extrusion of sulphur dioxide has been reported on irradiation of the 2,5-diaryl-1,4-dithiine 1,1-dioxides (156) to give the thiophenes (157) and (158) in low yields the exact stage at which sulphur dioxide is lost is uncertain, but the intermediacy of species such as (159) or (160) is supported by the formation of pyrrole derivatives in the presence of n-butylamine. 

We can apply this technique to another cheletropic reaction—the extrusion of sulphur dioxide from the sulphones (138 and 140).130 In this case, the reaction... 

Irradiation of the sulphone 68 at 147.0,123.6 and 106.7-104.8 nm results in decomposition. The principal step is the extrusion of sulphur dioxide and the formation of the biradical (69, R1=R2 = H) from which cyclopropane, propene and ethylene are... 

A novel extrusion of S02 is reported for the dithiine dioxides 104 to afford low yields of the thiophenes 105 and 106. The exact step at which the S02 loss occurs is not known but the proposed intermediates 107 and/or 108 have support from the formation of pyrrole derivatives when the reaction is carried out in the presence of n-butylamine103. Sunlight-induced extrusion of sulphur dioxide from the pyranone dioxide 109 is similar to the above and results in the formation of the reactive cyclopentadienone 110 which can be trapped by dienophiles or in their absence forms a trimer104. The platinum complex 111 is photochemically labile and decomposes into benzyne on irradiation105. 

Thiophen Dioxides and their Dihydro and Tetrahvdro Derivatives. - Flash vacuum pyrolysis of cobalt complexes (194) of thiophen 1 1-dioxides led to extrusion of sulphur dioxide and the formation of cyclobutadiene complexes (195). Stereochemical labelling experiments were employed in order to determine the mechanism of this transformation.114 A detailed publication on the reaction of... 

Nitrogen analogues (5) of o-xylylenes are produced as transient intermediates in the flash pyrolysis of o-amino-benzyl alcohols (4). They do not undergo ring-closure to benzazetines but cyclize to acridines via dihydroacridines. Possibly the same type of imine methide is involved in the thermal and photochemical extrusion of sulphur dioxide from the cyclic sulphones (6), which in this case does give the benzazetine. ... 

The thermal extrusion of sulphur dioxide from sulphones and sulpholene adducts is a well known procedure for producing 1,3- or 1,4-dienes and skipped polyenes. Lithium aluminium hydride has now been shown to effect this extrusion in good yield under mild conditions (refluxing ether)." The technique is similar to, but apparently mechanistically different from, the LiAlH4-promoted retro-Diels-Alder reaction of triazolinedione adducts reported recently by Barton. 

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